The Quantum Cheshire Cat: Can a Particle Be Separated from Its Properties?

Can a property of an object be physically separated from the object itself? Physicists have long theorized a tenet of quantum mechanics called the Quantum Cheshire Cat (named for the Alice in Wonderland character who famously leaves behind his wicked smile), in which neutrons can be separated from one of their properties, called the magnetic spin. Now, researchers have published the results of the first experimental evidence of this phenomenon.

In lieu of an electric charge, neutrons have a magnetic moment, or a neutron spin that can be influenced by magnetic fields. Physicists from the Vienna Institute of Technology used an experimental technique called neutron interferometry in order to determine whether the magnetic spin could be separated from the subatomic particle itself. This technique has previously been used to observe quantum superposition, or a particle's ability to exist in multiple theoretical states simultaneously prior to external observation.

From the paper: "If a quantum system is subject to a certain pre- and postselection, it can behave as if a particle and its property are spatially separated... The experimental results suggest that the system behaves as if the neutrons go through one beam path, while their magnetic moment travels along the other."

Generally, observations of this phenomenon have involved weak interaction, or interacting with the system in question so gently, one can avoid collapsing it from a quantum state into a classical state. But since the interaction is so weak, any results are confounded by the Heisenberg uncertainty principle. In order to glean meaningful experimental result, patterns must be observed using many particles.

In this experiment, a neutron interferometer splits a neutron beam into two parts, an upper and a lower beam. The upper neutron beam has a parallel spin to the many neutrons' direction of motion, while the lower beam's spin is in the opposite direction to the neutrons' motion. They then recombine the two beams and only observe the neutrons that have the a parallel spin to their direction of motion, or the ones that would have traveled through the upper beam. "This is called postselection," says team member Hermann Geppert. "The beam contains neutrons of both spin directions, but we only analyse part of the neutrons." Postselection is a quantum computing concept that allows quantum computers to yield answers by ignoring the many incorrect "trial and errors" that it would take to get there.

Because the quantum state is not collapsed, quantum phenomena such as interference still occur. So essentially, the experiment has the location of the neutrons interfere constructively in the upper beam and destructively in the lower beam, while the spin interferes conversely, guaranteeing that the location will be found in the upper beam and the spin will be found in the lower beam. This was demonstrated experimentally; a filter that would catch neutrons reduced the number of neutrons observed when applied to the upper beam, but made no difference when applied to the lower beam, while a magnetic field (expected to change the spin) only had an effect on the lower beam. But it is paradoxical that the magnetic field has any effect on the observed spins within the lower beam, as the neutrons supposedly never took that path. "By preparing the neurons in a special initial state and then postselecting another state, we can achieve a situation in which both the possible paths in the interferometer are important for the experiment, but in very different ways", said team member Tobias Denkmayr. "Along one of the paths, the particles themselves couple to our measurement device, but only the other path is sensitive to magnetic spin coupling. The system behaves as if the particles were spatially separated from their properties." In other words, the cat goes through the upper beam path, while its grin travels along the lower beam.

In the paper, the research team presents several perspectives on these results, including the notion that the spin cannot be considered a "real property of the system." Indeed, "it can be argued that the observable has no definite value between pre- and postselection."

This experiment could have a number of implications for high precision measurements, which are often confounded by specific properties of particles. According to team member Stephen Sponar, "When the quantum system has a property you want to measure and another property which makes the system prone to perturbations, the two can be separated using a Quantum Cheshire Cat, and possibly the perturbation can be minimized."